[CMake] Introduce a LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY flag.

On Darwin it is currently impossible to build LLVM with modules
because the Darwin system module map is not compatible with
-fmodules-local-submodule-visibility at this point in time.  This
patch makes the flag optional and off by default on Darwin so it
becomes possible to build LLVM with modules again.

http://reviews.llvm.org/D21827
rdar://problem/27019000

llvm-svn: 274196